17
CHAPTER 2
DESIGN AND DEVELOPMENT OF DOUBLE WINDING
INDUCTION MOTOR
2.1
INTRODUCTION
Conventional induction motor consists of only one set of winding
in its stator and DWIM consists of two sets of windings in the same stator. A
three phase supply is applied to one of the stator windings, a revolving
magnetic field is developed in the air gap and this field is shared by both
windings.
Two separate windings with displaced angle between them are
provided in the same stator for the improvement inefficiency. The behaviour
of an alternator with two three phase stator windings displaced by an angle is
analyzed by means of an orthogonal transformation (Fuchs 1974).
Weber (1992) presented a dual stator induction motor for energy
conservation which consists of two sets of RUN windings. Out of two sets of
windings, one set of RUN winding is energised to have sufficient MMF to
meet the reduced mechanical load, thereby the flux density in stator core
reduces, reduced eddy current losses and copper losses. Depending on the
shaft load of the machine, second set of RUN winding is excited through a
power source.
18
The main RUN winding is energized to have sufficient MMF to
meet the mechanical load. From the above discussion, it is learnt that most of
the methods suggested by researchers for double winding induction motors
are based on speed control applications and improvement of performance by
exciting both windings.
As further modification and improvement in energy conservation,
DWIM is suggested in this thesis. In the proposed model, in addition to
energy conservation, improvement in power factor and efficiency is also
obtained by loading both windings.
2.2
DESIGN OF DOUBLE WINDING INDUCTION MOTOR
In a double winding induction motor, when one of the windings is
connected to a three phase supply, a revolving magnetic field of constant
magnitude is developed in the air gap. This is utilized by both the stator
windings to work as induction motor to meet mechanical load while, a three
phase EMF is induced in the second set of winding to which electrical load
can be connected to work as an Induction alternator.
2.2.1
Representation of DWIM
Razik (2006) explained that the stator windings of double winding
induction motor can be arranged with different shift angles. In DWIM, shift
angle of 60 degrees or zero degrees are the best choice. In the proposed
model, to obtain optimum utilisation, both the windings are placed with zero
degree phase angle displacement between them and the representation of
proposed DWIM is shown in Figure 2.1.
19
Figure.2.1 Double Winding Induction Motor
2.2.2
Design Considerations
Design of the double winding induction motor is affected by
various constraints such as thermal limit, overload capacity and utility of
stator slots. Energy conserving double winding induction motor is ideal to be
used for low power operations due to the limitation in thermal insulation
value. The value of air gap flux density is large which determines large
overload capacity. Use of semi-enclosed slots results in silent operation. The
stator of DWIM consists of two sets of stator windings placed in the same slot
and therefore slot utility is increased. Slot utility factor for designed DWIM is
43.3% whereas for a conventional induction motor, it is about 29%.
2.2.3
Design of stator
Design procedure is presented for a 3kW, 415V, 50Hz, Double
Winding Induction Motor. Electrical loading, magnetic loading, efficiency
and power factor of the machine are chosen as given below:
Flux density in the stator core Bav
= 0.44 Wb/m2
Electrical loading
= 18000 ac/m
20
Efficiency
= 80 %
Power factor
= 0.85
Winding factor Kw
= 0.9
Output Co-efficient C0
= 11 x Bav x Kw x ac x 10-3
= 11 x 0.44 x 0.9 x18
= 83.2
kVA output of the motor Q
=3 / 0.8 x 0.85
= 4.41 kVA
The product of the diameter and the length of the core
D2 L
= Q / C0 x ns
= 4.41 / 83.2 x 25
= 2.12 x 10-3 m3
For good overall design L / ration is chosen as 1
Length of core L
= 0.785 D
Diameter of the core
= 0.139 m
Length of the core
= 0.11 m
DWIM presented in this thesis consists of two sets of identical coils in
the same stator core. Hence the design of one set is similar to the other.
Stator voltage per phase Vph
= 240 V
Flux per pole
= (Bav x x D x L) / P
m
= 5.28 mWb
Turns per phase
Tph
= Vph / (4.44 x 50 x 5.28 x 10-3 x 0.9)
21
= 228 turns
Slot per pole per phase
=3
Number of stator slots
= 36
Slot pitch
=
D / Ss
=
x 0.139 x 103 / 36
= 12 mm
Total stator conductors for each winding
= 3 x 2 x 228
= 1368 conductors
Conductors per slot for each winding
= 1368 / 36
= 38
Stator current per phase is
= 3x 103/ 3 x 240 x 0.8 x 0.85
= 6.1 A
18 SWG enameled copper coil is chosen for the winding
Diameter of the conductor
= 1.22 mm
Area of Stator conductor
= 1.17 mm2
Current density
= 6.1 / 1.17
s
= 5.21 A / mm2
Area of conductors per slot
= 1.17 x 76
= 88.92 mm2
Area of each slot
= 205 mm2
Space factor
= 88.92 / 205
= 0.433
22
2.3
PERFORMANCE OF DWIM
Bu et al (2003) presented a double winding induction machine and
its speed control methods. One stator acts as a motor and the other as a
generator. By controlling the voltage supplied to the generator winding, the
rotor speed can be adjusted. One of the stator winding of DWIM is connected
to a three phase supply, a revolving magnetic field is developed in the air gap.
With respect to first winding, this field interacts with rotor conductors and
develops torque to meet mechanical load, whereas a three phase EMF is
developed in second set of winding to which an additional electrical load can
be connected. The main scope of thesis is energy conservation, efficiency and
power factor improvement at reduced mechanical load of DWIM.
2.3.1
Load test with Electrical and Mechanical loads
In the proposed DWIM, both stator windings are of identical
nature. In order to obtain performance characteristics as conventional
induction motor, one of the windings is connected to a three phase supply
and other set of winding is left free. Load test has been carried out with
brake drum arrangement and electrical load on second set of winding to
study the performance of machine. Experimental set up with both electrical
and mechanical loading is shown in Figure 2.2. Table 2.1 shows the reading
observed considering one set of winding to operate the machine as induction
motor to meet the mechanical load and the second set of winding is
unloaded. Efficiency and power factor characteristic is shown in Figure 2.3.
The maximum efficiency of the machine is 83.6% and the corresponding
power factor is 0.63.
23
Figure 2.2 Experimental Setup
Speed in rpm
Power factor
% Slip
480
1466
0
0
0
0.27
2.3
415
3.0
720
1458
3.6
549
76.3
0.33
2.8
415
3.5
1240
1448
6.4
968
78.1
0.49
3.5
415
4.0
1680
1442
9.1
1368
81.4
0.58
3.9
415
4.5
2040
1438
11.3
1705
83.6
0.63
4.1
415
5.0
2250
1426
12.2
1814
81.6
0.63
4.9
415
5.5
2880
1416
15.7
2320
80.6
0.73
5.6
415
6.0
3360
1408
18.1
2672
79.5
0.78
6.1
% Efficiency
Input power (W)
2.5
Output power
(W)
Line current
415
Torque in Nm
Input voltage
Table 2.1 Conventional load test
24
Figure 2.3 Efficiency and power factor Characteristics (DWIM)
Test performance have been compared with IEC 60034-30, it is
observed that the standard efficiency for 3kW, 4- pole induction motor is
81.5%, but in tested machine efficiency is 79.9%. DWIM consists of two
identical windings, when a three phase 415V supply is applied to one
winding, the same magnitude of 415 V is induced in second winding. A lamp
load is used to test the machine whose power factor is unity. Table 2.2 shows
the reading observed with 1A electrical load on second set of winding in
addition to the mechanical load. Efficiency and power factor characteristic are
shown in Figure 2.4. The maximum efficiency of the machine is 86% and the
corresponding power factor is 0.78.
25
Total
% Efficiency
Power factor
% Slip
0
0
720
720
85
0.49
0.93
1380 1470
2.3
354
720
1074
78
0.64
1.60
3.5
1840 1468
5.6
859
720
1579
86
0.73
2.13
415
4.0
2240 1458
8.0
1220
720
1940
86
0.78
2.80
415
4.5
2760 1452
10.8
1640
720
2360
85
0.85
3.20
415
5.0
3080 1444
12.0
1810
720
2530
82
0.86
3.73
415
6.0
3640 1420
14.7
2185
720
2905
80
0.84
4.93
Input power (W)
Speed (rpm)
2.4
840
1480
415
3.0
415
Mechanical
(W)
Line current (A)
415
Torque (Nm)
Voltage (V)
output
Electrical output (W)
output (W)
Table 2.2 Mechanical load with 1A Electrical Load
Figure 2.4 Efficiency and power factor with 1A Electrical load (DWIM)
26
Table.2.3 shows the reading observed with 1A electrical load with
capacitor across the load and efficiency and power factor characteristics is
shown in Figure.2.5. When a capacitor of 20µF is included across the load,
the maximum efficiency increased to 85.8% and the corresponding power
factor is improved to 0.94.
Input Voltage
Line Current (A)
Input Power (W)
Speed in rpm
Torque Nm
Mechanical
Output (W)
Electrical
Output (W)
Total
Output (W)
% Efficiency
Power Factor
Table 2.3 Mechanical load with 1A Electrical Load and Capacitance
415
2.7
1800
1440
6.6
994
480
1474
81.8
0.93
415
3.1
2080
1435
8.0
1207
520
1727
83.0
0.93
415
3.6
2440
1435
10.5
1573
520
2093
85.8
0.94
415
4.1
2780
1420
12.4
1838
520
2358
84.8
0.94
415
4.7
3160
1410
14.2
2097
520
2617
82.8
0.94
415
5.4
3660
1400
16.7
2446
520
2966
81.1
0.94
Figure 2.5
Efficiency and power factor with 1A load and capacitor
(DWIM)
27
Table 2.4 shows the reading observed with 2A electrical load on the
second set of winding in addition to the mechanical load and the
corresponding performance characteristics is shown in Figure 2.6. The
maximum efficiency of the machine is 84% and the corresponding power
factor is 0.85.
Input Voltage
Line Current (A)
Input Power (W)
Speed in rpm
Torque in Nm
Mechanical
Output (W)
Electrical
Output (W)
Total
Output (W)
% Efficiency
Power Factor
Table 2.4 Mechanical Load with 2A Electrical Load
415
3.0
1560
1488
0
0
1200
1200
77
0.72
415
4.0
2360
1472
4.9
756
1200
1956
83
0.82
415
4.5
2680
1464
7.9
1203
1200
2403
90
0.83
415
5.0
3040
1452
8.8
1342
1200
2542
84
0.85
415
5.5
3440
1438
10.8
1624
1200
2824
82
0.87
Figure 2.6 Efficiency and power factor with 2A Electrical load (DWIM)
28
Table 2.5 shows the reading observed with 3 A electrical load on
the second set of winding in addition to the mechanical load and the
corresponding performance characteristics is shown in Figure 2.7. The
maximum efficiency of the machine is 92% and the corresponding power
factor is 0.91.
Electrical
Output (W)
Total
Output (W)
% Efficiency
Power Factor
0
0
2125
2125
86
0.93 1.7
415 4.0 2640 1460
2.0
301
2125
2426
92
0.91 2.7
415 4.5 3120 1452
3.7
715
2125
2840
91
0.96 3.2
415 5.0 3480 1440
6.6
1035
2125
3160
90
0.97 4.0
415 5.5 3920 1430
9.4
1410
2125
3535
90
0.99 4.7
415 6.1 4360 1426 11.6
1728
2125
3853
88
099
% Slip
Mechanical
Output (W)
415 3.7 2480 1474
Speed in rpm
Torque Nm
Input Power (W)
Line Current (A)
Input Voltage
Table 2.5 Mechanical load with 3A Electrical Load
4.9
Figure 2.7 Efficiency and power factor with 3A Electrical load (DWIM)
29
Table 2.4 corresponds to the reading with 2A electrical load in the
second winding and Table 2.5 shows the reading with 3A electrical load in the
second winding. The efficiency and power comparison is shown in Figure 2.8.
Main focus of this thesis is to improve efficiency and power factor when the
machine is lightly loaded with mechanical output. When operated as a
conventional induction motor for a load current of 3A, efficiency is 76.3%
and power factor 0.33. For the same load current, electrical load of 720 W is
added in the second winding and hence efficiency is improved to 78% and
power factor to 0.64. With a capacitor of 20 µF across the load, efficiency is
improved to 83% and power factor to 0.93.
Figure 2.8 Efficiency and power factor comparison for 2A and 3A (DWIM)
2.3.2
Modes of operation
A controller circuit has been designed to operate DWIM in power
balancing and maximum efficiency modes of operation. Initially, mode of
operation is selected. In Mode I operation, controller is designed to operate
the machine at its maximum efficiency, but at its rated capacity in Mode II of
operation.
Contactors are energised through relay coils and a controller to
connect electrical load to second set of stator coils. The operation of DWIM
30
for the two modes of operation is explained using a flow chart shown in
Figure 2.9. The load current corresponding to maximum efficiency is 4.5A
and load current corresponding to rated capacity of the machine is 6.1A.
These values are arrived from load test results.
The control circuit for power balancing mode of operation is shown in
Figure 2.10. The contactor arrangement for Mode I and Mode II operation is
shown are Figure 2.11 and Figure 2.12. PIC controller senses the load current
due to the mechanical load and switches ON the external electrical load to the
second set of winding in order to maintain total current of 4.5A, corresponds
to maximum efficiency.
Figure 2.9 Flowchart for modes of operation (DWIM)
31
Figure 2.10 Control circuit for power balancing mode of operation (DWIM)
Figure 2.11 Contactor arrangements for Mode I operation (DWIM)
32
Figure 2.12 Contactor arrangements for Mode II operation (DWIM)
A current transformer is coupled with a controller through a
precision rectifier. Depending upon load current in the first winding,
controller switches external electrical load through contactors C1, C2 and C3
in different combination. In power balancing mode of operation, controller is
programmed such a way that the machine runs at its rated capacity. Contactor
operation for different loading condition is shown in Table 2.6. The
specification of PIC IC 16F877 is given below:
8K x 14 words of Flash Program Memory
368 x 8 bytes of Data Memory (RAM)
256 x 8 bytes of EEPROM data memory
Wide operating voltage range: 2.5V to 5.5V
High Sink/ Source current :25 mA
USART/SCI with 9-bit address detection.
33
Table 2.6 Contactor operation
Load current Corresponds
to first winding
Contactor C1
Contactor C2
Contactor C3
(1A load)
(1 A load)
(0.5A load)
2.0<I<3.5
ON
ON
ON
3.5< I <4.5
OFF
ON
ON
4.5< I <5.5
OFF
OFF
ON
When load current is between 2A and 3.5A, then all the three
contactors will be in closed condition such that additional load of 2.5 A can
be added to the second set of winding. When the load current is between
3.5 A and 4.5 A, then C4 and C5 contactor will be closed such that additional
load of 1.5 A is added to the second set of winding. Similarly, when the load
current is between 4.5 A and 5.5 A, then C5 contactor alone will be closed
such that 0.5 A is added to the second set of winding.
2.4
SUMMARY
Double Winding Induction Motor consists two sets of winding in
the same stator core. If the machine is operated as conventional induction
motor, for a load current of 3.0 A, the efficiency and power factor are 76.3%
and 0.33 respectively, then an electrical load of 720 W is connected in the
second set of winding. Its efficiency is improved to 78% and power factor to
0.64. When a capacitor of 20 F is included in the load side and 520W load in
the second set of winding its efficiency still improves to 83% and power
factor to 0.93. By an appropriate control circuit, DWIM can be operated in
power balancing mode or maximum efficiency mode.
In this work, energy conservation is obtained by operating the
machine at its rated capacity or at maximum efficiency using control circuit.
34
In addition to performance improvement, energy conservation is also obtained
by loading both windings. Example for energy conservation is given below:
Considering load test results with 1A electrical load in second set of winding
Supply voltage
: 415 V
Line current
: 4.5 A
Input power
: 2760 W
Output power
: 1640 W (Mechanical)
Output power
: 720 W (Electrical)
In the above test result, electrical power of 720 watts has been
tapped from second winding. If the machine is operated for 10 hours a day,
without depending on separate supply, monthly energy conservation from the
same machine will be 0.72X 10 X 30 = 216 kWhrs.
DWIMs would be an alternative choice for rewound motors and
induction motors operated continuously. By providing an additional load in
second set of winding, in addition to efficiency and power factor
improvement, energy conservation is possible. Thus, DWIM provides
opportunity for energy conservation and performance improvement.